import java.util.Scanner;

public class text_5 {
    public static void main(String[] args) {
        System.out.println("输入购买长度");
        Scanner sc=new Scanner(System.in);
        double a=sc.nextDouble();
        if (a%3==0){
            int b=(int)(a/3);
            System.out.println("需要买："+b);
        }else{
            int c=(int)(a/3+1);
            System.out.println("需要买："+c);
        }
    }
}
